In the manufacture of catalytic converter monoliths, the monolith substrate is normally formed of extruded ceramic material or stacked corrugated metal foil so as to have many passages extending therethrough for exhaust gas flow past a catalyst deposited on the passages walls. The many passages form a large surface area within a small cross-sectional envelope that is normally further minimized significantly for both space and material savings by treating the surface with a ceramic wash coat such as alumina so as to form a coating thereon of greatly expanded surface area to serve as the actual base for the noble metal catalytic material. However, the normal amount of washcoat then reduces the flow area significantly (e.g. by as much as 25%). And thus, it will be appreciated that an excess of washcoat could unnecessarily increase the engine back pressure significantly, while insufficient washcoat could significantly reduce the long term effectiveness of the catalytic action.
In the manufacturing process, the object then is to apply the correct or proper amount of ceramic coating to the monolith substrate passages; i.e. an amount that allows proper catalytic action without unduly increasing the engine back pressure. The proper amount to apply can be precisely determined but the problem is in the accurate application which to date by all methods known can result in a significant number of out of tolerance substrates. For example, there may be too much or too little coating, the passages may be tapered because of nonuniform coating caused by "sagging" or "running" of the coating prior to drying, and/or the coating may have plugged some of the passages. Since the performance of the converter is highly dependent on the proper amount and distribution of the washcoat, it is necessary to determine whether the desired amount of coating was applied, whether it was applied uniformly along the passages, and whether any passages were blocked. Moreover, it is desirable that the inspection be made rapidly without loss of accuracy that there be 100% inspection available and that the method be non-destructive of the monolith substrates as well as not require their special clean up. The present methods typically involve computation based on coating weight, and measurements by pressure drop and radiogauging and as a result are deficient in various respects.